It all depends on what you use these applications for. For now, TikTok is an entertainment platform with the ability to promote your other social networks. If you have your own brand or business, you still have to keep Facebook or Instagram. All because they're loved by advertisers due to the predictability of the platforms. TikTok is random, and there's no way to predict if a post will have any kind of reach at all. Even trendy TikTok Influencers are constantly asking to subscribe to other social networks if you pay attention. It's all about the money.
The popularity of Facebook is all about money, and there is a lot of it.